A polarizing plate is an optical member included in a liquid crystal display (LCD). The polarizing plate may have a multilayer structure of a polarizing element comprising an iodine compound or a dichroic polarizing material, which is arranged in a given direction, and a protective film to protect the element and to be formed on one or both sides thereof. In addition, the polarizing plate may further comprise an additional functional film such as a retardation plate, a wide view angle compensation plate or a brightness enhancing film.
Each film consisting of a multilayer polarizing plate is made of materials having molecular structures and compositions different from each other. Therefore, each film in the polarizing plate represents physical properties different from each other. Especially, there is a problem that dimensional stability is lowered depending on difference of shrinkage or expansion behavior by each material in the polarizing plate under a severe condition such as high temperature or high humidity condition.
For example, when the polarizing plate is fixed by a pressure sensitive adhesive, stress is focused on the protective film, and the like, by the above difference of behavior under the severe condition to cause birefringence and a light leakage phenomenon called as a light beaming phenomenon.
As a method for solving such problems, a technique to provide the pressure sensitive adhesive with stress relaxation characteristic has been known. Specifically, a method of designing the pressure sensitive adhesive to have high creep against an external stress and to be easily modified has been known (ex. Korean Unexamined Patent Publication No. 1998-079266, Japanese Unexamined Patent Publication No. 2002-047468).
However, in the above technique, the pressure sensitive adhesive has very low tailoring property and workability. If the tailoring property and workability are lowered as such, there are problems that on mass-producing optical films such as polarizing plates, defects, such as extrusion or pressing of the pressure sensitive adhesive, are caused, so that yield decreases highly.
Furthermore, there is an attempt to minimize light leakage by designing the pressure sensitive adhesive very hard.
For example, in Japanese Unexamined Patent Publication Nos. 2007-197659 and 2007-212995, a technique of preparing a pressure sensitive adhesive is disclosed, which is prepared by adding a multifunctional acrylate, an isocyanate hardener and a photoinitiator to an acrylic polymer containing a carboxyl group, followed by UV irradiation. In Japanese Unexamined Patent Publication No. 2007-212995, a technique of preparing a pressure sensitive adhesive is disclosed, which is prepared by combining a copolymer containing a hydroxyl group and a copolymer containing a carboxyl group in a given ratio, and adding a multifunctional acrylate, a multifunctional isocyanate hardener and a photoinitiator thereto, followed by UV irradiation.
However, said pressure sensitive adhesives have very low initial adhesive strength depending on storage modulus (G′), so that durability under high temperature or high humidity is lowered, or much light leakage is caused.